High-curvature lenses are used as object lenses for light information recording/regenerating apparatuses (light pickup apparatuses) and semiconductor-developing apparatuses. Typical examples of the high-curvature lenses are those having large numerical apertures (NA). Recently proposed are light pickup apparatuses comprising object lenses having NA of 0.85, which are used for laser beams having wavelength of 405 nm. A lens having NA of 0.85 has a shape shown in FIG. 3, for instance. When used as an object lens, laser beams enter into the lens through a surface R1 and exits from a surface R2. Substantially parallel laser beams are vertically incident to the object lens at a center. Accordingly, the light has an extremely large incident angle in a peripheral portion of the lens. In the example shown in FIG. 3, the maximum incident angle of light is 65° in an effective-diameter region E of the lens. Although incident light desirably passes through the object lens efficiently, the amount of reflected light increases proportionally to the incident angle. FIG. 4 shows the reflectance of a lens having the shape shown in FIG. 3 at a surface R1, which is made of glass having a refractive index of 1.72. 15.5% of the incident light is reflected by the lens at a position at which the incident angle is 65°.
To reduce the amount of reflected light to achieve efficient transmission, object tenses for light pickup apparatuses and semiconductor-developing apparatuses are provided with anti-reflection coatings. For instance, a single-layer, anti-reflection coating is designed to have such thickness that the light path difference between light reflected at a surface of the anti-reflection coating and light reflected at a boundary between the anti-reflection coating and the lens is odd times a half of the wavelength, so that these reflected lights are cancelled by interference. The single layer, anti-reflection coating is designed to have a refractive index smaller than that of a lens and larger than those of incident media such as air, etc. It is said that an anti-reflection coating formed on a lens made of glass having a refractive index of about 1.5 ideally has a refractive index of 1.2-1.25. However, there is no material having such an ideal refractive index. Accordingly, MgF2 having a refractive index of 1.38 is widely used as a material for the anti-reflection coating.
Anti-reflection coatings made of inorganic materials such as MgF2 are conventionally formed by a vacuum vapor deposition method, a sputtering method, a CVD method, etc. However, the anti-reflection coating formed by these methods is generally thinner on a peripheral portion of the lens than on a center portion thereof. In general, the optical thickness D(θ′) of the anti-reflection coating at an incident angle θ′ is represented by the following formula (4):D(θ′)=D0·(cos θ′)x  (4),wherein D0 is the optical thickness of the anti-reflection coating at a lens center, and x is a constant of 0-1. When the anti-reflection coating is formed by a vacuum vapor deposition method, x is about 0.7. Thus, the optical thickness is deviated from the designed thickness in a peripheral portion of the lens, and sufficient anti-reflection characteristics cannot be obtained because of a large incident angle as described above, resulting in an extremely large amount of reflected light. Accordingly, such optical element suffers from the problem that although there is high transmission in a center portion, there is insufficient transmission in the entire element. In addition, to form the anti-reflection coating by a vapor deposition method, a vacuum apparatus is needed, resulting in high production cost.
JP 2005-173029A proposes an optical element comprising an anti-reflection coating designed to (a) minimize the sum of light reflected in each region obtained by dividing a lens surface in an effective-diameter region, or (b) maximize the sum of light transmitted through each region. This optical element has high transmission as a whole, but the use of a conventional anti-reflection coating made of MgF2 having as relatively high a refractive index as 1.38 fails to provide satisfactory anti-reflection characteristics.
The formation of fine roughness on a lens surface is known to provide the lens with anti-reflection characteristics. Specifically, a lens is directly worked by a chemical treatment method such as etching, and/or a physical treatment method such as mechanical roughening, light lithography, etc., or roughness pattern on a mold surface is transferred to the lens. However, these methods are usable only on low-curvature lenses.
JP 6-167601A discloses a method for producing a porous anti-reflection coating comprising the steps of etching a substrate surface, vapor-depositing a mixed layer of SiO2 and NaF at a volume ratio NaF/SiO2) of 1-3 on the surface, and immersing the mixed layer in water. The immersion of the mixed layer of SiO2 and NaF in water causes NaF to be dissolved in water to form fine pores, resulting in a porous anti-reflection coating Of SiO2. The porous anti-reflection coating obtained by this method has a refractive index of about 1.3. However, this porous anti-reflection coating has large hygroscopicity, resulting in water entering into fine pores during use to change the refractive index, etc. Thus, it suffers from a serious problem of deterioration with time.
JP 2001-272506A discloses a method for producing an anti-reflection coating comprising the steps of forming an alkyl group-containing layer on a substrate at a low temperature using a CVD method, and subjecting the layer to a heat treatment to remove alkyl groups form the layer to form fine pores. Specifically, a silica layer is formed by the reaction of alkyl amines, etc. having large polarity with alkoxysilanes such as tetraisocyanate silane, etc., and the silica layer is heated to 300° C. or higher to remove the alkyl groups. This forms fine pores having diameters of less than 10 nm in the silica layer, thereby providing the silica layer with a refractive index of about 1.25. The heat treatment improves the hydrophobicity of the porous silica layer, making deterioration with time less likely. However, to provide the silica layer with sufficient hydrophobicity, heating should be conducted at higher than 400° C. Some molded glass lenses used as object lenses for light pickup apparatuses have glass transition temperatures lower than 400° C., and the method described in JP 2001-272506A cannot be applied to such lenses.
JP 2005-234447A proposes an optical member having fine roughness on the surface, which comprises an anti-reflection coating formed by treating a zinc-compound-containing gel layer with a water-containing liquid at a temperature of 20° C. or higher. JP 2005-275372A proposes an optical member having fine roughness on the surface, which comprises an anti-reflection coating formed by treating an alumina-containing gel layer with hot water. However, JP 2005-234447A and JP 2005-275372A disclose only optical lenses for spectacles, camera finder lenses, prisms, fly-eye lenses and toric lenses, but fail to describe the formation of anti-reflection coatings containing the above zinc compounds or alumina on high-curvature lenses such as object lenses for light pickup apparatuses, etc.